1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting device having improved device characteristics.
2. Description of the Related Art
For example, a nitride semiconductor is used in a blue LED employed as a light source for illumination, backlight, or the like and in LED, LD, and the like used for multi-colorization. Due to the difficulty of manufacturing a bulk single crystal, growth of GaN on a heterogeneous substrate, such as sapphire and SiC, by using a Metal Organic Chemical Vapor Deposition (MOCVD) method is carried out. Being excellent in terms of stability in a high-temperature ammonia atmosphere in an epitaxial growth process, a sapphire substrate is especially used as a growth substrate. A sapphire substrate is an insulating substrate, and a nitride semiconductor on a sapphire substrate is etched after epitaxial growth until an n-type gallium nitride layer is exposed, an n-type contact is formed on the etched surface, and two electrodes, which are a p-type electrode and an n-type electrode, are formed on the same surface side.
On the other hand, a nitride semiconductor light emitting device having a structure in which two electrodes, which are a p-type electrode and an n-type electrode, are arranged opposite to each other has been proposed. For example, as illustrated in FIG. 12, a p-type GaN layer 22, an InGaN active layer 23, an n-type GaN layer 24 are stacked on a p electrode 21. Here, an n electrode is formed on a center portion of the n-type GaN layer 24 so as to achieve a structure in which the n electrode is arranged opposite to the p electrode 21; however, the n electrode is not illustrated. Furthermore, the upper direction of the drawing is a direction of light extraction.
Light generated in the InGaN active layer 23 is emitted in 360 degree direction. In order to increase an amount of light to be extracted to the outside as much as possible, the lower surface of the p electrode 21 formed of metal is used as a reflecting mirror, and light emitted to the lower side is reflected by the p electrode 21 so as to be extracted to the upper side.
However, even if a reflecting mirror is used as described above, due to total reflection occurring at an interface between the n-type GaN layer 24 and an atmospheric layer, the light extraction efficiency is extremely low. Total reflection occurs at a boundary surface in the case where light goes from a medium having a large refractive index towards a medium having a small refractive index, and occurs when an incident angle of light entering the boundary surface is a critical angle or larger.
A range of light entering a boundary surface at an angle smaller than a critical angle below which no total reflection occurs is indicated by a light extraction cone 25. Light, such as reflected light from the p electrode 21 and light directly going upwardly from the InGaN active layer 23, which comes into the range of the light extraction cone 25 proceeds as shown by an arrow of a solid line T into the atmosphere, and then is extracted. However, light which does not come into the range of the light extraction cone 25 causes total reflection, as shown by a solid line R, at an interface between the n-type GaN layer 24 and the atmospheric layer, and becomes light which cannot be extracted.
Especially, the refractive index of GaN (approximately 2.5) is extremely large compared to the refractive index of air (approximately 1.0). Accordingly, when a difference in refractive index is increased, the range of the light extraction cone 25 is decreased; therefore, the light extraction efficiency is deteriorated.
In the above-described conventional structure of nitride semiconductor light emitting device, the light extraction efficiency can not be improved. Therefore, as described in Japanese Patent Publication Application No. 2006-310893, for example, in order to improve the light extraction efficiency, a technique to form asperities in a part of a semiconductor layer is proposed.
This is, as shown by a broken line in FIG. 12, to form a cone-shaped projection 26 on the surface of the n-type GaN layer 24 serving as a light extraction surface by etching processing or blast processing. With the cone-shaped projection 26, for example, light, indicated by the solid line R, which is used to be totally reflected at a boundary surface proceeds inside of the cone-shaped projection 26, and its incident angle into the side surface of the cone-shaped projection 26 becomes smaller than the critical angle. Accordingly, the light is emitted into the atmosphere as shown by a broken line S, and the light extraction efficiency is improved.
However, in order to form the cone-shaped projection 26, etching processing or blast processing is applied on the surface of the n-type GaN layer 24. Accordingly, due to the processing, there is a problem that the n-type GaN layer 24 is damaged and the voltage-current characteristics are affected.
Furthermore, as shown in FIG. 12, in the case where light is extracted not from the n-type GaN layer side but from the p side, it is necessary to perform etching processing or blast processing on a p-type GaN-based semiconductor layer, such as a p-type GaN layer, so as to form a cone-shaped projection. Processing damage on a p-type GaN-based semiconductor layer is larger than that on an n-type GaN-based semiconductor layer. Since processing damage causes defect which exhibits n-type conduction, device leakage current is increased, or an ohmic junction between an electrode and a p-type GaN-based semiconductor layer is made difficult because of GaN becoming highly resistive.
In the meantime, as another example of a nitride semiconductor light emitting device, a ridge waveguide-type nitride semiconductor laser having a ridge stripe structure is known. In order to mount and electrically connect the semiconductor laser device onto a substrate, a flip-chip bonding method is employed. Flip-chip bonding is, as described in Japanese Patent Publication Application No. 2007-5473, for example, to bond an electrode on a ridge part and an electrode on a submount by soldering or the like by placing a surface on the side of the ridge part formed on a p-type nitride semiconductor down.
A ridge waveguide-type nitride semiconductor laser device is configured, for example, as shown by a solid line part in FIG. 22. An n-type nitride semiconductor layer 82, an active layer 83, a p-type GaN-based semiconductor layer 84 are sequentially stacked on a conductive substrate 81, and a convex-shaped ridge stripe part B is formed on the p-type GaN-based semiconductor layer 84. A p electrode 85 is formed at a top of the ridge stripe part B, while an n electrode 87 is formed on a rear surface of the conductive substrate 81. The p electrode 85 is flip-chip bonded with an electrode, a wiring pattern, or the like on a supporting substrate 86.
In a semiconductor laser device having such a configuration, a stripe width of the ridge stripe part B is formed to be narrow for narrow injection of electric current; therefore, the ridge stripe part B is susceptible to damage. For example, in a process to scribe or divide a wafer, if a surface in which the ridge stripe part B is formed is caused to come into contact with a stage or the like or is applied with pressure, the ridge stripe part B may be damaged.
Furthermore, when the flip-chip bonding is carried out, a certain pressure from the n electrode 87 side is applied so as to establish a bond with the supporting substrate 86. Therefore, stress is concentrated in the ridge stripe part B, and damage occurs.
As described above, when the ridge stripe part Bis damaged, due to a decrease in light emitting properties and light emission intensity, and the like, reliability of the laser device is deteriorated as a result. Furthermore, the nitride semiconductor laser is likely to incline to the supporting substrate 86 when bonded thereto; therefore, this becomes a cause for lowering the device characteristics.
Furthermore, an inclination direction of and an inclination angle of a nitride semiconductor laser when a bond with the supporting substrate 86 is established are different for each nitride semiconductor laser device. Accordingly, due to the inconsistency in inclination, heat-releasing properties vary; therefore, this also becomes a cause for creating a variation in the light emitting properties.
Therefore, as shown by a dotted line in FIG. 22, it is also conceivable to form ridge parts 90 and 91, which have a ridge stripe structure similar to that of the ridge stripe part B, with metal or an insulating film. It is assumed that, in this method, compared to the case where metal is embedded in all grooves on both sides of the ridge stripe part B, transverse stress due to a difference in heat expansion coefficient can be reduced to some degree. However, the fact remains that stress applied onto the ridge stripe part B due to the difference in heat expansion coefficient occurs in both a longitudinal direction and a transverse direction. Therefore, damage on the ridge stripe part B and a decrease in the light-emitting properties are caused.